Goal:
  
  A : Type
  p : A → bool
  n : nat
  x : A
  Hnth : nth_error [] n = Some x
  HPl : is_true (forallb p [])
  ============================
   (locked false = true)


DEBUG:Going to execute:
Ltac.assumption
DEBUG LTAC Evaluated term: not_locked_false_eq_true